1. Field of the Invention
This invention relates to methods and apparatus for preparing gas mixtures of predetermined composition.
2. Use and Preparation of Gas Mixtures
Accurately determined gas mixtures (the term gas as used herein includes vapors) are in widespread use as calibration or reference standards for analytical instrumentation and as feed stock for scaled down chemical reactions or processes, particularly in research and development. Thus, there is a need for mixtures of component gases in precisely predetermined proportions, and a corresponding need for methods and apparatus for the accurate production of such gaseous blends.
Commercial bottled gas and vapor mixtures of known composition are widely available for use in analytical instrumentation. The proportion of each gas component in a bottle is usually fixed when the gas is pressurized at the bottling plant, but for certain applications it is convenient for a user to mix specified quantities of two or more bottled gases to obtain a gas mixture having a specified predetermined composition different from that of the bottled gases. This composition is preferably variable by the user according to local need.
Currently available methods for the local preparation of gas mixtures by a user incorporate mass flow and permeation techniques. These methods typically use dynamic devices in which blending occurs only when gas components are flowing. These techniques are relatively inaccurate and lend themselves to applications requiring few gas components in the mixture. Indeed, some of the devices are only effective for producing specific mixtures. An example of this technique is disclosed in U.S. Pat. No. 3,948,281, incorporated herein by reference.
In contrast to the above methods, commercial preparation of gas mixtures is usually by a gravimetric technique, in which a gas-containing vessel is weighed, or by a partial pressure technique. For several reasons, neither of these methods is adequate for the preparation of calibration gas blends suitable as reference standards for modern analytical instruments. The accuracy of the gravimetric method, for example, is dependent to a large degree on the weight of each component relative to the total weight of the vessel and its contents. This results in lower accuracies being attained in low density mixtures, such as hydrogen and helium, and also in situations where the components of interest are in low concentration.
The partial pressure method also has limited accuracy because the high pressures required to make the process commercially feasible can compound errors caused either by the absence of precise temperature control during compression, expansion and blending of the gases, or inhomogeneities in the resulting intermediate and final gas mixtures. Consequently, the partial pressure method of blending gases has not been found to be particularly suitable for the preparation of accurately measured gas blends, as for calibration gases.
A survey of topics related to production of gas mixtures is presented in the 1975 copyrighted book Gas Mixtures--Facts and Fables by Frank Scarporoicer, available from Matheson Gas Products Company, 932 Paterson Plank Road, P.O. Box 85, East Rutherford, N.J. 07073; the book is hereby incorporated by reference for indicating the background of the invention and illustrating the state of the art.
3. Preparation of Calibration Gas Mixtures
Gas mixtures for instrument calibration frequently contain very low concentrations of one or more component gases and are particularly difficult to prepare accurately. Their manufacture often requires elaborate procedures, some of which are suitable only for specific compounds. For example, gas permeation, which is a dynamic technique, requires precision temperature control; it is sensitive to active component life and has a narrow range of applicability. In general, gas mixtures prepared with high concentrations of one or more component gases together with low concentrations of one or more other gases must be analyzed in a separate procedure to confirm the desired concentration of each component gas.
U.S. Pat. No. 4,142,860, incorporated herein by reference, discloses an apparatus for the production of calibration gas mixtures. The apparatus includes a pressurized gas mixing vessel of large thermal inertia relative to that of the component gases blended therein, this unit serving to aid in the control of temperature fluctuations. The apparatus also includes a movable stirring element disposed within the mixing vessel to aid in mixing gases which are added to the vessel from a plurality of externally located pressurized tanks via a plurality of inlet valves mounted on the vessel. A pressure transducer measures the gas pressure in the mixing vessel, and an indicator displays the pressure reading.
In operation, the mixing vessel can be purged and evacuated, the component gases then being added seriately and stirred to produce a calibration gas mixture. One problem with this apparatus is that the inlet valves contain significant gas volume which is substantially isolated from the gas mixing action of the stirring element.
U.S. Pat. No. 4,254,797, incorporated herein by reference, discloses an apparatus for the production of calibration gas mixtures which is improved over that shown in the '860 patent. The apparatus described in the '797 patent has proven satisfactory for blending component gases in accurately measured quantities, but it includes a closed loop gas path for providing gases to the mixing vessel. This closed loop path contains spaces wherein small amounts of gas can remain sequestered even after the system had been purged, thereby reducing the accuracy of the gas blending system. Additionally, a pressure transducer location remote from the mixing vessel results in substantial isolation of a portion of the contained gas from the gas mixing action of the stirring element.
Thus, substantial elimination of gas sequestration, together with improved mixing and increased accuracy and repeatability, would be desirable improvements in existing apparatus for producing calibration gas mixtures.